Hybrid seed, that is, seed produced by hybridization or cross-fertilization of closely related plants, can be grown into progeny hybrid plants possessing “hybrid vigor” or a desirable combination of traits not possessed by either parent plant (which are typically inbred plants). Hybrid plants can display superior agronomic performance characteristics, including improvement of plant size, yield, nutritional composition, disease resistance, herbicide tolerance, stress (heat, cold, drought, nutrient, salt) tolerance, climatic adaptation, and other desirable traits.
Efficient hybrid seed production requires that cross-pollination predominates over self-pollination. A major limitation in the production of hybrid seed for many crop species is the lack of simple, reliable and economical methods of generating sterility in at least one parent (especially the male parent, to result in male-sterility while leaving female gametes intact and accessible for pollination by a suitable pollen donor). Male sterility is also useful where pollen spread is not desirable, e.g., from a domestic plant to its wild relatives, or where flower fertilization is not desirable, e.g., in the case of ornamental flowers which deteriorate in condition after pollination.
Male sterility can be accomplished, for example, by physical removal of the organs containing the male gametes. In some species, this is a straightforward although labor-intensive and therefore expensive process (e.g., detasselling in maize). In other species, such physical emasculation is difficult because of the plant's anatomy. Alternative techniques that do not involve manual or physical emasculation could provide substantial economic savings.
Chemical gametocides have been also described as a method of generating male-sterile plants. Typically, such a chemical gametocide is an herbicidal compound that when applied to a plant at an appropriate developmental stage or before sexual maturity is capable of killing or effectively terminating the development of a plant's male gametes while leaving the plant's female gametes, or at least a significant proportion of them, capable of under going cross-pollination. For example, glyphosate tolerance has been genetically engineered into corn (U.S. Pat. No. 5,554,798), and use of the herbicide glyphosate (N-phosphonomethylglycine) as a gametocide, and transgenic plants that are vegetatively- and female-tolerant of glyphosate but male-sensitive to glyphosate, are disclosed in U.S. Pat. No. 4,735,649 and in PCT International Patent Application Publication WO99/46396A2. However, the levels of glyphosate necessary to kill most of the male gametes while leaving a sufficient number of female gametes still capable of fertilization often resulted in stunting or chlorosis of the plants. Thus, a major drawback of using glyphosate as a gametocide, as is generally true with most chemical gametocides, is the phytotoxic side effects resulting from lack of sufficient selectivity for gametes.
Commercial production of hybrid seed using chemical gametocides is limited primarily by their lack of selectivity for gametes in general. Compounds that possess some selectivity in targeting gametes to a greater extent than vegetative tissues are generally non-discriminating regarding the sex of the gametes destroyed. Thus, methods for improving the selectivity of a chemical gametocide would be highly desirable. Even more desirable would be methods that provide a first parent plant that is male-sterile, and a second parent plant that is female-sterile, thus ensuring that the seed produced are the result of hybridization between the two parent plants, and not of self-fertilization.
This invention provides methods of producing hybrid seed, and additionally provides recombinant DNA constructs, transgenic plant chromosomes, cells, plants, and seeds containing such constructs useful in these methods. The recombinant DNA constructs, transgenic plant chromosomes, cells, plants, and seeds, and methods for their use in making hybrid seed provide a greatly improved way to use herbicides as chemical gametocides. The recombinant DNA constructs of this invention include an exogenous microRNA recognition site, allowing expression of a messenger RNA encoding a protein imparting tolerance to an herbicide to be controlled by a microRNA endogenous to a plant in which the recombinant DNA construct is transcribed.
MicroRNAs (miRNAs) are non-protein coding RNAs, generally of between about 19 to about 25 nucleotides (commonly about 20-24 nucleotides in plants), that guide cleavage in trans of target transcripts, negatively regulating the expression of genes involved in various regulation and development pathways (Bartel (2004) Cell, 116:281-297). In some cases, miRNAs serve to guide in-phase processing of siRNA primary transcripts (see Allen et al. (2005) Cell, 121:207-221).
Some microRNA genes (MIR genes) have been identified and made publicly available in a database (“miRBase”, available on line at microma.sanger.ac.uk/sequences). The applicants have disclosed novel MIR genes, mature miRNAs, and miRNA recognition sites in U.S. patent application Ser. No. 11/303,745, filed 15 Dec. 2005. Additional MIR genes and mature miRNAs are also described in U.S. Patent Application Publications 2005/0120415 and 2005/144669A1. MIR genes have been reported to occur in inter-genic regions, both isolated and in clusters in the genome, but can also be located entirely or partially within introns of other genes (both protein-coding and non-protein-coding). For a recent review of miRNA biogenesis, see Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385. Transcription of MIR genes can be, at least in some cases, under promotional control of a MIR gene's own promoter. MIR gene transcription is probably generally mediated by RNA polymerase II (see, e.g., Aukerman. and Sakai (2003) Plant Cell, 15:2730-2741; Parizotto et al. (2004) Genes Dev., 18:2237-2242), and therefore could be amenable to gene silencing approaches that have been used in other polymerase II-transcribed genes. The primary transcript (which can be polycistronic) termed a “pri-miRNA”, a miRNA precursor molecule that can be quite large (several kilobases) and contains one or more local double-stranded or “hairpin” regions as well as the usual 5′ “cap” and polyadenylated tail of an mRNA. See, for example, FIG. 1 in Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385.
In plant cells, microRNA precursor molecules are believed to be largely processed in the nucleus. In plants, miRNAs and siRNAs are formed by distinct DICER-like (DCL) enzymes, and in Arabidopsis a nuclear DCL enzyme is believed to be required for mature miRNA formation (Xie et al. (2004) PLoS Biol., 2:642-652). Additional reviews on microRNA biogenesis and function are found, for example, in Bartel (2004) Cell, 116:281-297; Murchison and Hannon (2004) Curr. Opin. Cell Biol., 16:223-229; and Dugas and Bartel (2004) Curr. Opin. Plant Biol., 7:512-520. MicroRNAs can thus be described in terms of RNA (e.g., RNA sequence of a mature miRNA or a miRNA precursor RNA molecule), or in terms of DNA (e.g., DNA sequence corresponding to a mature miRNA RNA sequence or DNA sequence encoding a MIR gene or fragment of a MIR gene or a miRNA precursor).
MIR gene families are estimated to account for 1% of at least some genomes and capable of influencing or regulating expression of about a third of all genes (see, e.g., Tomari et al. (2005) Curr. Biol., 15:R61-64; G. Tang (2005) Trends Biochem. Sci., 30:106-14; Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385). Because miRNAs are important regulatory elements in eukaryotes, including animals and plants, transgenic suppression of miRNAs could, for example, lead to the understanding of important biological processes or allow the manipulation of certain pathways (e.g., regulation of cellular differentiation, proliferation, and apoptosis) useful, for example, in biotechnological applications. See, for example, O'Donnell et al. (2005) Nature, 435:839-843; Cai et al. (2005) Proc. Natl. Acad. Sci. USA, 102:5570-5575; Morris and McManus (2005) Sci. STKE, pe41 (stke.sciencemag.org/cgi/reprint/sigtrans;2005/297/pe41.pdf). MicroRNA (MIR) genes have identifying characteristics, including conservation among plant species, a stable foldback structure, and processing of a specific miRNA/miRNA* duplex by Dicer-like enzymes (Ambros et al. (2003) RNA, 9:277-279). These characteristics have been used to identify miRNAs and their corresponding genes in plants (Xie et al. (2005) Plant Physiol., 138:2145-2154; Jones-Rhoades and Bartel (2004) Mol. Cell, 14:787-799; Reinhart et al. (2002) Genes Dev., 16:1616-1626; Sunkar and Zhu (2004) Plant Cell, 16:2001-2019). Publicly available microRNA genes are catalogued at miRBase (Griffiths-Jones et al. (2003) Nucleic Acids Res., 31:439-441).
MiRNAs are expressed in very specific cell types in Arabidopsis (see, for example, Kidner and Martienssen (2004) Nature, 428:81-84, Millar and Gubler (2005) Plant Cell, 17:705-721). Suppression can be limited to a side, edge, or other division between cell types, and is believed to be required for proper cell type patterning and specification (see, e.g., Palatnik et al. (2003) Nature, 425:257-263). Suppression of a GFP reporter gene containing an endogenous miR171 recognition site was found to limit expression to specific cells in transgenic Arabidopsis (Parizotto et al. (2004) Genes Dev., 18:2237-2242). Recognition sites of miRNAs have been validated in all regions of an mRNA, including the 5′ untranslated region, coding region, and 3′ untranslated region, indicating that the position of the miRNA target site relative to the coding sequence may not necessarily affect suppression (see, e.g., Jones-Rhoades and Bartel (2004). Mol. Cell, 14:787-799, Rhoades et al. (2002) Cell, 110:513-520, Allen et al. (2004) Nat. Genet., 36:1282-1290, Sunkar and Zhu (2004) Plant Cell, 16:2001-2019).
The mature miRNAs disclosed herein are processed from MIR genes that generally belong to canonical families conserved across distantly related plant species. These MIR genes and their encoded mature miRNAs are also useful, e.g., for modifying developmental pathways, e.g., by affecting cell differentiation or morphogenesis (see, for example, Palatnik et al. (2003) Nature, 425:257-263; Mallory et al. (2004) Curr. Biol., 14:1035-1046), to serve as sequence sources for engineered (non-naturally occurring) miRNAs that are designed to silence sequences other than the transcripts targetted by the naturally occurring miRNA sequence (see, for example, Parizotto et al. (2004) Genes Dev., 18:2237-2242; also see U.S. Patent Application Publications 2004/3411A1 and 2005/0120415), and to stabilize dsRNA. A MIR gene itself (or its native 5′ or 3′ untranslated regions, or its native promoter or other elements involved in its transcription) is useful as a target gene for gene suppression (e.g., by methods of the present invention), where suppression of the miRNA encoded by the MIR gene is desired. Promoters of MIR genes can have very specific expression patterns (e.g., cell-specific, tissue-specific, or temporally specific), and thus are useful in recombinant constructs to induce such specific transcription of a DNA sequence to which they are operably linked.
This invention provides methods for producing hybrid seed, using recombinant DNA constructs including recognition sites corresponding to novel mature miRNAs having specific expression patterns in crop plants. The recombinant DNA constructs of the invention transcribe to RNA including: (a) at least one exogenous miRNA recognition site recognizable by a mature miRNA that is specifically expressed in reproductive tissue of the plant; and (b) messenger RNA encoding a protein imparting tolerance to an herbicide. These constructs are useful for making and using transgenic plant chromosomes, cells, plants, and seeds, including inducibly sterile transgenic plants, useful, e.g. in producing hybrid seed.